Micromechanical component with different doping types so that one type is anodized into porous silicon

ABSTRACT

A micromechanical component having a substrate made from a substrate material having a first doping type, a micromechanical functional structure provided in the substrate and a cover layer to at least partially cover the micromechanical functional structure. The micromechanical functional structure has zones made from the substrate material having a second doping type, the zones being at least partially surrounded by a cavity, and the cover layer has a porous layer made from the substrate material.

FIELD OF THE INVENTION

The present invention relates to a micromechanical component having asubstrate made from a substrate material having a first doping, amicromechanical functional structure provided in the substrate and acover layer to at least partially cover the micromechanical functionalstructure. The present invention also relates to a method formanufacturing a micromechanical component.

BACKGROUND INFORMATION

Micromechanical function is understood to include active function, e.g.,a sensor function, or passive function, e.g., a printed conductorfunction.

Although it may be applied to any micromechanical component andstructure, such as, for example, sensors and actuators, an exemplaryembodiment according to the present invention and the underlying problemare elucidated with reference to a micromechanical component, e.g., anacceleration sensor, which may be manufactured, for example, using asilicon surface micromachining technology.

Monolithically integrated inertial sensors produced by surfacemicromachining technology, in which the sensitive movable structures aresituated on the chip without protection (analog devices), may result inincreased expenses for handling and packaging.

This problem may be circumvented by a sensor having an evaluationcircuit on a separate chip, e.g., covering the structures produced bysurface micromachining with a second cap wafer. This type of packagingmay constitute a large share of the cost of manufacturing anacceleration sensor by surface micromachining. These costs may arise,for example, as a result of the high surface area required between thecap wafer and the sensor wafer and due to structuring (2-3 masks, bulkmicromechanics) of the cap wafer.

The structure of a functional layer system and a method for the hermeticcapping of sensors using surface micromachining is referred to in GermanPublished Patent Application No. 195 37 814, in which the production ofa sensor structure is explained. The cited hermetic capping is performedusing a separate cap wafer of silicon, which may be structured usingexpensive structuring processes, such as KOH etching. The cap wafer isapplied to the substrate with the sensor (sensor wafer) using a sealglass. This requires a wide bonding frame around each sensor chip toensure an adequate adhesion and seal integrity of the cap. This maylimit the number of sensor chips per sensor wafer. Due to the largeamount of space required and the expensive production of the cap wafer,sensor capping may incur considerable costs.

FIG. 10 is a schematic cross-sectional view of a micromechanicalcomponent.

As shown in FIG. 10, a semiconductor substrate is identified as 10, asacrificial layer as SL, a functional level having a micromechanicalfunctional structure (e.g., an acceleration sensor) as FS, a seal glassas SG, a cavity as CA and a cap wafer as CW. As described above, thecorresponding manufacturing process may be expensive since it requirestwo wafers, for example, a substrate wafer 10 and a cap wafer CW, whichmay be adjusted to each other.

The production of a cavity under a porous silicon layer is referred toin G. Lammel, P. Renaud, “Free-standing mobile 3D microstructures ofporous silicon,” Proceedings of the 13^(th) European Conference onSolid-State Transducers, Eurosensors XIII, The Hague, 1999, pages535-536.

SUMMARY OF THE INVENTION

It is believed that an exemplary micromechanical component andmanufacturing method according to the present invention allow a simpleand cost-effective manufacturing of a micromechanical component, e.g.,an acceleration sensor, a micropump, a flow channel, a check valve, aflow regulator, etc., using porous substrate material.

The use of such porous substrate material, e.g., porous silicon, maypermit simple production of a cavity having a superimposed diaphragm inone process step. The micromechanical structures may be produced in thesame process step. Thus, it is believed that advantages of an exemplarymicromechanical component according to the present invention and anexemplary method for manufacturing the same include:

the production of micromechanical structures in a cavity having asuperimposed diaphragm in one process step;

the exclusion of the cap wafer with wafer-to-wafer adjustment;

the inclusion of a vacuum in the cavity; and

the production of structures having complex depth profiles.

An exemplary embodiment according to the present invention is based onthe micromechanical functional structure having zones made from thesubstrate material having a second doping, the zones being at leastpartially surrounded by a cavity, and the cover layer having a porouslayer made from the substrate material. During manufacturing, thep-doped zones may be readily etched, when the substrate is anodized.However, the n-doped zones may not be etched or only their surfaces maybe insignificantly etched.

According to an exemplary embodiment of the present invention, a sealinglayer seals the pores of the porous layer. In this manner, apredetermined atmosphere under the diaphragm may be set.

According to another exemplary embodiment of the present invention, thesealing layer has an oxide layer formed on the porous zone.

According to still another exemplary embodiment of the presentinvention, at least one of the zones made from the substrate materialhaving the second doping type has a supporting zone to support theporous zone.

According to yet another exemplary embodiment of the present invention,at least one of the zones made from the substrate material having thesecond doping type is completely detached from its surroundings.

According to still another exemplary embodiment of the presentinvention, the cavity includes a flow channel, which may be connected byat least two back openings.

According to yet another exemplary embodiment of the present invention,the back openings are each connected by one transfer opening, which isformed in the zone.

According to still another exemplary embodiment of the presentinvention, a sealing layer seals the pores of the porous layer and adetection device situated on the sealing layer piezoresistively detectsthe flow rate.

According to yet another exemplary embodiment of the present invention,a check valve device is provided above a corresponding transfer openingwithin the flow channel, the check valve device having at least one ofthe zones made from the substrate material having the second dopingtype, which is detached from its surroundings or is resilientlyconnected to the substrate material.

According to still another exemplary embodiment of the presentinvention, two check valve devices of different dimensions are providedabove a corresponding transfer opening, a sealing layer sealing thepores of the porous layer and the porous zone, the sealing layer beingoperable as a pump diaphragm.

According to yet another exemplary embodiment of the present invention,the cavity includes a circular inner flow channel and a concentric outerflow channel, which are connected by radial ports in a separation zonemade from the substrate material having the second doping type, theinner flow channel being interrupted by a bar and a back inlet openingbeing provided on one side of the bar and a first back outlet openingbeing provided on the other side of the bar and a second back outletopening being provided in the outer flow channel, so that a medium,flowing through the back inlet opening, may be separated, specific tomass, by centrifugal force, through the first and second back outletopening.

According to still another exemplary embodiment of the presentinvention, the substrate has at least one trench, which is partiallyfilled with a doping material of the second doping type and partiallyfilled with a filler.

According to yet another exemplary embodiment of the present invention,the substrate material is silicon.

According to still another exemplary embodiment of the presentinvention, the zones made from the substrate material having the seconddoping type are provided in the substrate at different depths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 c are cross-sectional views of a micromechanical componentmanufactured using an exemplary manufacturing method for manufacturingthe micromechanical component according to the present invention.

FIG. 2 is a micromechanical structure manufactured using anotherexemplary manufacturing method according to the present invention.

FIGS. 3a and 3 b are cross-sectional views of a micromechanicalcomponent manufactured using another exemplary manufacturing methodaccording to the present invention.

FIGS. 4a-4 d are cross-sectional views of a micromechanical componentmanufactured using yet another exemplary manufacturing method accordingto the present invention.

FIGS. 5a and 5 b are cross-sectional views of an exemplarymicromechanical component according to the present invention.

FIGS. 6a and 6 b are cross-sectional views of a second exemplarymicromechanical component according to the present invention.

FIGS. 7a and 7 b are cross-sectional views of a third exemplarymicromechanical component according to the present invention.

FIGS. 8a and 8 b are cross-sectional views of a fourth exemplarymicromechanical component according to the present invention.

FIGS. 9a and 9 b are cross-sectional views of a fifth exemplarymicromechanical component according to the present invention.

FIG. 10 is a cross-sectional view of a micromechanical component.

DETAILED DESCRIPTION

In the figures, identical symbols denote identical or functionallyequivalent components.

FIGS. 1a-1 c are schematic views in cross-section of an exemplarymicromechanical component manufactured using manufacturing method formanufacturing the micromechanical component according to the presentinvention.

As shown in FIG. 1a, the micromechanical component includes a p-dopedwafer substrate 10 made of silicon, n-doped zones 15 in substrate 10, ametal mask 20 and metal mask openings 21.

First, the n-doped zones 15 are produced in p-doped substrate 10, forexample, using standard semiconductor processes, such as an implantationmethod, in which the penetration depth with a corresponding distributionmay be determined by adjusting the energy. The n-doped zones 15 aresituated at a specific depth below the substrate surface and may, forexample, also be situated on the substrate surface (not shown).

Then, parts of the substrate surface are masked using metal mask 20. Anitride mask, an oxinitride mask, etc., may be used instead of the metalmask 20.

As shown in FIG. 1b, the n-doped zones 15 of substrate 10 defined bymask 20 are etched electrochemically by hydrofluoric acid (HF) to makethem porous. The porosity is controlled by the current density.Initially, a low current density is applied, resulting in the productionof a layer of low porosity. The current density is then raised above acritical value. In addition, the hydrofluoric acid concentration may bereduced, or other solutions that inhibit H₂ formation may be used. As aresult, the pores in the lower zone of a porous layer 30 become sized,so that the substrate material is essentially or entirely etched awayand a cavity 50 is formed under the remaining porous layer 30. In thiscase, the term electropolishing is used. The material is removed throughporous layer 40.

The structure formed in the functional level by n-doped zone 15 includesexposed structures 60, permanent structures 70 and structural elements,which are connected to porous layer 30 by a supporting zone 40, thusforming a diaphragm support. Depending on the width of the n-dopedstructures, the structures may also be undercut and exposed asexemplified by exposed structures 60 of FIG. 1b.

An exemplary manufacturing method according to the present invention mayconsider the different dopings, n and p, for example, reactingdifferently to the electrochemical etching attack in semiconductorsubstrate 10. For example, the p-doped zones in semiconductor substrate10 may be anodized well. However, the n-doped zones 15 may resist theetching attack. Consequently, the buried n-doped zones 15 may not beattacked during the anodizing. A porous film, which may superficiallyform on n-doped zones 15, may be eliminated by tempering in H₂ or by ashort dip in silicon-etching solutions, such as, e.g., TMAH or solutionscontaining KOH. In this case, the etch front passes around n-doped zones15.

As shown in FIG. 1c, the pores of porous silicon zone 30, which form anupper limit of cavity 50, are sealed by different processes. Thedeposition of a layer with oxide, nitride, metal, epitaxy or theoxidation of porous layer 30 to form sealing layer 75 are exemplaryarrangements. Tempering in H₂, for example, at temperatures above 1000°C., may also result in a vacuum-tight seal. The pressure ratios duringthe sealing process determine the internal pressure arising in cavity50, and H₂ may diffuse out by tempering.

The structure exemplified in FIG. 1c may be used as an accelerationsensor. Exposed structures 60 may be capable of vibrating in transverseaccelerations, as a result of which the distance between exposedstructures 60 and permanent structures 70 may periodically change. Thechange in distance may be analyzed capacitively by an interdigitalcapacitor. If a vacuum is to be enclosed under the sealing diaphragmmade up of porous zone 30 and sealing layer 70, the sealing diaphragmmay be stabilized by cited supporting zones 40.

Alternatively, all micromechanical structures manufactured using anexemplary method according to the present invention may be producedtogether with a corresponding integrated circuit, e.g., an evaluationcircuit. For this purpose, an epitaxy layer may be deposited on theporous zone. The corresponding circuit components may be produced, forexample, using CMOS, bipolar or mixed processes.

FIG. 2 is a micromechanical structure manufactured using anotherexemplary manufacturing method according to the present invention.

As shown in FIG. 2, reference symbol 200 denotes a doping mask and 201denotes a doping mask opening. In contrast to metal mask 70 of FIGS.1a-1 c, an n-doping is used as mask 200 in this exemplary embodiment.The combination of an n-doping as a mask and an additional mask layer onthe doped substrate surface, e.g., nitride, may also be used.

FIGS. 3a and 3 b are cross-sectional views of a micromechanicalcomponent manufactured using another exemplary manufacturing methodaccording to the present invention.

As shown in FIG. 3, n-doped zones 15 a, 15 b are provided at differentdepths, made possible by the selection of different implantationenergies. As a result, structures having very complex depth profiles maybe produced. As shown in FIGS. 3a and 3 b, two different implantationsare performed to produce the upper functional level having n-doped zones15 a and to produce the functional level having n-doped zones 15 b. Inother respects, the method steps occur in a similar manner, describedabove with reference to FIGS. 1a-1 c.

The second functional level may be incorporated by depositing an epitaxylayer, into which the second functional level is implanted after thefirst functional level has been implanted.

FIGS. 4a-4 d are cross-sectional views of a micromechanical componentmanufactured using yet another exemplary manufacturing method accordingto the present invention.

In addition to the reference symbols previously introduced, FIG. 4ashows trenches 80 in p-doped semiconductor substrate 10. The trenches 80may be introduced in semiconductor substrate 10, for example, using anetching method in combination with a hard mask.

As shown in FIG. 4b, after the trenches 80 have been created, a chemicalvapor deposition occurs with an n-doped deposition layer 90, e.g.,epitaxial silicon, to form n-doped zones 15 c. Subsequently, as shown inFIG. 4c, the trenches 80 are filled with a filler, e.g., polysilicon,and the resulting structure is planarized. Finally, an epitaxialpolysilicon layer 150 is deposited as shown in FIG. 4d.

This procedure of trench formation, doping, filling and epitaxialdeposition may be repeated cyclically to produce complex depth profiles.For example, this exemplary method according to the present inventionmay permit a very high-definition doping profile to be produced with ahigh aspect ratio. In addition to n-doped polysilicon, for example,oxide, BPSG, etc., may be used for filling. For example, filler 100 maybe n-doped or p-doped, depending on the intended appearance of theresulting structure.

Following the exemplary procedure shown in FIG. 4d, the further processsteps described above with reference to FIGS. 1b and 1 c occur.

FIGS. 5a and 5 b are cross-sectional views of an exemplarymicromechanical component according to the present invention.

FIGS. 5a and 5 b illustrate a branched flow channel having definedtransfer openings. In this exemplary embodiment according to the presentinvention, the transfer openings are provided as back openings 510,while porous zone 30 is hermetically sealed by a sealing layer 75.N-doped zones 15 define the lower limit of cavity 50 a and thus thebottom of the flow channel. The Y-shaped structure of the flow channelmay be attained by suitable masking.

For example, transfer openings 520 may be provided in the structureshown in FIGS. 5a and 5 b, which are provided in n-doped zone 15, sothat, when back openings 510 are etched from the back, the passages donot become too large, which is indicated by the correspondingbell-shaped back etching profile. In this respect, n-doped zone 15 alsoacts as an etching stop for the etching from the back.

In the exemplary embodiment according to the present invention describedwith reference to FIGS. 5a and 5 b, an additional epitaxy layer (notshown) may be deposited and power components (not shown), e.g., powertransistors, may be implemented on the additional epitaxy layer. Theflow channel may then carry a coolant liquid or a coolant gas or anothercoolant, so that the power components may be cooled from the back bythermal contact. Compared with cooling from the front, cooling from theback may not require the surface to be protected from the coolant. Theflow channel may have a meandering shape or may be entwined in anotherdirection for this application (not shown).

FIGS. 6a and 6 b are cross-sectional views of a second exemplarymicromechanical component according to the present invention.

In this exemplary embodiment according to the present invention,piezoresistive resistors 630, 630′ are provided on the sealing layerabove porous zone 30. Varying flow rates in flow direction FD result ina varying pressure, which subjects the diaphragm and thus piezoresistiveresistors 630, 630′ to a voltage of varying strength. The resultingchange in resistance may be analyzed. A heating structure havingtemperature sensors analogous to the previous thermal mass flow sensorsmay also be used.

It is believed to be advantageous in that, due to the supply of the massflow from the back, it may not be necessary to protect resistanceelements 630, 630′ against media.

FIGS. 7a and 7 b are cross-sectional views of a third exemplarymicromechanical component according to the present invention.

An exemplary embodiment according to the present invention shown inFIGS. 7a and 7 b, relates to a check valve, and includes a micro-sealingball 730 and a micro-sealing plate 740 which, together with transferopening 720 in n-doped zone 15 b, form a check valve. Micro-sealing ball730 and/or micro-sealing plate 740 are formed simultaneously with theflow channel and/or transfer opening 720 during the anodization processand seal off transfer opening 720 in the event of a return flow.

FIGS. 8a and 8 b are cross-sectional views of a fourth exemplarymicromechanical component according to the present invention.

The exemplary embodiment described with reference to FIGS. 8a and 8 b isa micropump. The diaphragm contains porous zone 30, and sealing layer 70is thinner and may be deflected in direction DD.

A deflection may be implemented, for example, by using a magnetic layeras sealing layer 75, which may be deflected by an electromagnet. Thediaphragm may be thermally or electrostatically deflected. In doing so,cavity 50 d is enlarged or reduced in volume, and the use of twodifferent check valves 830, 830′ permit a flow direction FD to beimposed. Check valve 830 has the shape of a ball, and check valve 830′,in the form of an ellipsoid, which interacts with an elliptical,elongated opening.

When the diaphragm is deflected upwards, check valve 830′ closes theright inlet, and liquid may flow past the check valve. Thus, liquid isdrawn into the left transfer opening. With a downward deflection, leftcheck valve 830 closes the round transfer opening, while liquid may flowpast right check valve 830′. Thus, the liquid drawn in is pressed outthrough the right transfer opening.

FIGS. 9a and 9 b are cross-sectional views of a fifth exemplarymicromechanical component according to the present invention.

The exemplary structure according to the present invention and describedwith reference to FIGS. 9a and 9 b is a gas centrifuge. The gascentrifuge includes a circular inner flow channel 50 e and a concentricouter flow channel 50 f, which are connected by radial ports 905 in aseparation zone 15 made of the substrate material. The inner flowchannel is interrupted by a bar 910. A back inlet opening I is locatedon one side of the bar and a first back outlet opening O1 is provided onthe other side of bar 910. A second back opening O2 is provided at theend of outer flow channel 50 f. Thus, a medium flowing through backinlet opening I may be routed to first or second back outlet opening O1,O2 specific to mass by centrifugal force. That is, heavier gascomponents are pressed into outer flow channel 50 f due to thecentrifugal force, while the lighter gas components remain in inner flowchannel 50 e. To intensify the affected separation effect, a pluralityof such gas centrifuges may, for example, be connected in series, oneafter the other.

It should be noted that the present invention is not limited to thevarious exemplary embodiments described above, but rather may bemodified in a variety of ways.

For example, micromechanical base materials such as, e.g., germanium,may be used instead of the silicon substrate. Also, sensor structuresmay be formed.

What is claimed is:
 1. A micromechanical component comprising: asubstrate made from a first substrate material having a first dopingtype, the substrate including a micromechanical functional structure;and a cover layer at least partially covering the micromechanicalfunctional structure; wherein the micromechanical functional structureincludes at least one zone made from a second substrate material havinga second doping type, the at least one zone is at least partiallysurrounded by a cavity, and the cover layer includes a porous layer madefrom the substrate material.
 2. The micromechanical component of claim1, wherein at least one of the at least one zone includes a supportingzone for supporting the porous layer.
 3. The micromechanical componentof claim 1, wherein at least one of the at least one zone is one ofcompletely detached from surroundings and resiliently connected to thesubstrate.
 4. The micromechanical component of claim 1, wherein thesubstrate includes at least one trench partially filled with a dopingmaterial having the second doping type and at least partially filledwith a filler.
 5. The micromechanical component of claim 1, wherein thefirst and second substrate materials include silicon.
 6. Themicromechanical component of claim 1, wherein the zones of the substrateare situated at different depths of the substrate.
 7. Themicromechanical component of claim 1, wherein the cavity includes avacuum.
 8. The micromechanical component of claim 1, further comprising:a scaling layer to seal pores of the porous layer.
 9. Themicromechanical component of claim 8, wherein the sealing layer includesan oxide layer formed on the porous layer.
 10. The micromechanicalcomponent of claim 8, further comprising: a detection device forpiezoresistively detecting a flow rate, the detection device being onthe sealing layer.
 11. The micromechanical component of claim 8, furthercomprising: two check valve devices having different dimensions, each ofthe check valve devices situated above a respective one of transferopenings, the porous layer with the sealing layer being operable as apump diaphragm.
 12. The micromechanical component of claim 8, wherein apredetermined atmosphere is set by the sealing layer.
 13. Themicromechanical component of claim 8, further comprising: at least onepiezoresistive resistor arranged on the sealing layer above the porouslayer.
 14. The micromechanical component of claim 1, wherein the cavityincludes a flow channel connected to at least two back openings.
 15. Themicromechanical component of claim 14, wherein the at least two backopenings are connected by respective transfer openings, and each of therespective transfer openings are formed in the zone.
 16. Themicromechanical component of claim 15, further comprising: a check valvedevice in the flow channel and above a corresponding one of therespective transfer openings within the flow channel, the check valvedevice including at least one of the zones.
 17. The micromechanicalcomponent of claim 1, wherein the cavity includes a circular inner flowchannel and a concentric outer flow channel connected by radial ports ina separation zone made from the second substrate material, the innerflow channel is interrupted by a bar, a back inlet opening is arrangedon one side of the bar, a first back outlet opening is arranged on another side of the bar, and a second back outlet opening is arranged inthe outer flow channel, so that a medium flowing through the back inletopening may be separated in accordance with a mass of the medium by acentrifugal force through the first and second back outlet openings.